Photolithography is a process where a pattern in a mask is transferred to a substrate through alignment, exposure and development. In the case of multiple layers, it is of importance to ensure that the patterns of different layers are aligned with each other. An overlay deviation refers to a positional deviation between the patterns of different layers due to e.g. misalignment of the masks.
FIGS. 1A and 1B are plan views of overlay keys used in measuring overlay accuracy between different layers in the prior art. As shown in FIG. 1A, a plurality of marks 201 are formed in a first layer that are rectangular frames (by way of example, only three overlay marks 2011, 2012 and 2013 are shown). Then, as shown in FIG. 1B, in forming subsequent layers above the first layer, marks 2021, 2022 and 2023, which are shown as black blocks, are formed in turn in respective layers. Specifically, mark 2021 is formed in a second layer, mark 2022 is formed in a third layer, and mark 2023 is formed in a fourth layer. Marks 2021, 2022 and 2023 are located within the rectangular frames 2011, 2012 and 2013, respectively. When overlay accuracy is measured, the coordinates of the centers of the respective black blocks and the coordinates of the centers of the respective rectangular frames may be determined in a pre-established coordinate system. The overlay accuracy between the first layer and a layer to which a black block corresponds may be measured by calculating a difference between the coordinate of the center of the black block and the coordinate of the center of a corresponding rectangular frame.
A problem with such a prior art overlay key however is that 1) the plurality of rectangular frames occupy a relatively large area, wasting the space of the substrate, and that 2) the overlay accuracy between any two layers other than the first layer cannot be measured directly.
Thus, there is a need for an improved overlay key, a method of forming the overlay key, and a method of measuring overlay accuracy.